I've only owned fourth gen Firebirds for the past 20 years, so I have no personal experience, but I would vote for the C5. Timeless beauty, pop-up headlights, and the LS1.
There ain’t much room for ppl taller than 4’11 lol there’s room for a seat for kids tho. I only mustang I ever need was a 2007 v6 and same with that one